Influence of sea temperature and initial marine feeding on survival of Atlantic salmon Salmo salar post-smolts from the Rivers Orkla and Hals, Norway

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid‐norway (1 sea‐winter, SW, fish) and River Hals in north Norway (1–3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post‐smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river–fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post‐smolt stomachs in Trondheimsfjord. The abundance of returning S.salar was, however, neither correlated to forage ratio (R_F) nor other prey groups in post‐smolt stomachs in the two fjord systems. In the Altafjord, the post‐smolts fed mainly on pelagic fish larva (70–98%) and had a stable R_F (0·009–0·023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in R_F (0·003–0·036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post‐smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1–3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post‐smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post‐smolt phase.     

Manipulation of the energetic state of Atlantic salmon Salmo salar juveniles and the effect on migration speed

Atlantic salmon Salmo salar smolts were produced with similar energetic states as wild S. salar and the effect of low energetic state on smolt migration was tested. The total energetic state of the fish (body lipids and proteins) in the spring was correlated with Fulton's condition factor (K). Fish at a low energetic state swam slower but migrated further than fish at a higher energetic state when tested in two experimental streams. During a period of starvation throughout the winter and spring, fish conserved their body‐lipid reserves at 1·5% by using more protein as an energy source and the metabolic shift occurred between 3·5 and 1·5% body lipids. An energetic state of approximately 3·5 kJ g^?1 (K ≈ 0·65) appeared to be the critical limit for survival.     

Passing a seawater challenge test is not indicative of hatchery‐reared Atlantic salmon Salmo salar smolts performing as well at sea as their naturally produced conspecifics

Despite satisfactory reactions to seawater challenge tests indicative of appropriate physiological state, hatchery‐reared Atlantic salmon Salmo salar smolts stocked in the Eira River in Norway between 2001 and 2011 performed less well at sea in terms of growth, age at maturity and survival than smolts of natural origin. The mean rates of return to the river for hatchery‐reared and naturally produced S. salar were 0·98 and 2·35%. In the Eira River, c. 50 000 hatchery‐reared S. salar smolts of local origin were stocked annually to compensate for reduced natural smolt production following regulation for hydroelectric purposes, while a mean of 17 262 smolts were produced naturally in the river. This study demonstrates that, although captive S. salar perform well in seawater challenge tests, hatchery‐reared smolts are not necessarily as adaptable to marine life as their naturally produced counterparts. These findings suggest that production of hatchery‐reared smolts more similar to naturally produced individuals in morphology, physiology and behaviour will be necessary to improve success of hatchery releases. Where possible, supplementary or alternative measures, including habitat restoration, could be implemented to ensure the long‐term viability of wild stocks.     

Interaction between migration behaviour and estuarine mortality in cultivated Atlantic salmon Salmo salar smolts

Migration behaviour and estuarine mortality of cultivated Atlantic salmon Salmo salar smolts in a 16 km long estuary were studied using two methods: (1) acoustic telemetry and (2) group tagging in combination with trap nets. Progression rates of surviving individuals through the estuary were relatively slow using both methods [0·38 L_T (total length) s^?1 v. 0·25 L_T s^?1]. In 2012, the progression rate was slow from the river to the estuary (0·55 L_T s^?1) and the first part of the estuary (0·31 L_T s^?1), but increased thereafter (1·45–2·21 L_T s^?1). In 2013, the progression rate was fast from the river to the estuary (4·31 L_T s^?1) but was slower thereafter (0·18–0·91 L_T s^?1). Survival to the fjord was higher in 2012 (47%) compared to 2013 (6%). Fast moving individuals were more likely to migrate successfully through the estuary compared to slower moving individuals. Adult recapture of coded‐wire‐tagged S. salar was generally low (0·00–0·04%). Mortality hot spots were related to topographically distinct areas such as the river outlet (in 2012) or the sill separating the estuary and the fjord (in 2013). At the sill, an aggregation of cod Gadus morhua predating on cultivated smolts was identified. The results indicate that slow progression rates through the estuary decreases the likelihood of smolts being detected outside the estuary. The highly stochastic and site‐specific mortality patterns observed in this study highlight the complexity in extrapolating mortality patterns of single release groups to the entire smolt run of wild S. salar.     

Changes in swimming depths of Atlantic salmon Salmo salar post‐smolts relative to light intensity

Eight hatchery‐reared Atlantic salmon Salmo salar post‐smolts, implanted with acoustic depth sensing transmitters and manually tracked for 5–12 h in the Hardangerfjord (Norway), spent most of their time (49–99%) at 1–3 m depth during the day, whereas four of seven fish tracked were found close (<0·5 m) to the surface at night, with a strong negative cross‐correlation between general swimming depth and surface light intensity. Hence, the actual swimming depth of post‐smolts during their early marine migration may depend on the light conditions, although the individual variation in vertical movement pattern was large. No cross‐correlations were found between light intensity and swimming depth during daytime periods with rapid changes in light intensity, indicating that other factors than light intensity were important in initiating the irregular dives that were recorded down to 6·5 m depth.     

Long‐term effects of photoperiod,temperature and their interaction on growth,gill Na+, K+‐ATPase activity,seawater tolerance and plasma growth‐hormone levels in Atlantic salmon Salmo salar

This study was undertaken to examine the long‐term effects of photoperiod, temperature and their interaction on growth, gill Na⁺,K⁺‐ATPase (NKA) activity, seawater tolerance and plasma growth‐hormone levels in Atlantic salmon Salmo salar pre‐smolts and smolts. The fish (mean ± s.e . initial body mass = 15·9 ± 0·4 g) were reared on two photoperiods (continuous light, LL, and simulated natural photoperiod, LDN, 60° 25′ N) and two temperatures (8·3 and 12·7° C) from June to May of the following year. Mean body mass was affected by photoperiod, temperature and their interactions. Both temperature groups on LL developed peak levels in gill NKA activity from October to November, 4–5 months prior to the natural season for the parr–smolt transformation. Fish at 12° C showed peak levels in NKA activity 4–6 weeks before the fish at 8° C. Fish in all four experimental groups showed maximum NKA activity within a similar size range (113–162 g). The present findings further indicate that smoltification in S. salar is to some extent driven by size, and that S. salar will develop smolt characteristics, e.g. a marked increase in NKA activity, within a similar size range. Faster‐growing S. salar will, thus, reach this size threshold at a relatively younger age.     

Quantification of ventricular β2‐adrenoceptor density and ligand binding affinity in wild sockeye salmon Oncorhynchus nerka smolts using a novel modification to the tritiated ligand technique

A new, image‐based, tritiated ligand technique for measuring cardiac β₂‐adrenoceptor (β₂‐AR) binding characteristics was developed and validated with adult rainbow trout Oncorhynchus mykiss hearts so that the tissue limitation of traditional receptor binding techniques could be overcome and measurements could be made in hearts nearly 14‐times smaller than previously used. The myocardial cell‐surface (functional) β₂‐AR density of O. nerka smolts sampled at the headwaters of the Chilko River was 54·2 fmol mg protein^?1 and about half of that previously found in return migrating adults of the same population, but still more than twice that of adult hatchery O. mykiss (21·1 fmol mg protein^?1). This technique now opens the possibility of investigating cardiac receptor density in a much wider range of fish species and life stages.     

Migratory behaviour and survival rates of wild northern Atlantic salmon Salmo salar post‐smolts: effects of environmental factors

To study smolt behaviour and survival of a northern Atlantic salmon Salmo salar population during river descent, sea entry and fjord migration, 120 wild S. salar were tagged with acoustic tags and registered at four automatic listening station arrays in the mouth of the north Norwegian River Alta and throughout the Alta Fjord. An estimated 75% of the post‐smolts survived from the river mouth, through the estuary and the first 17 km of the fjord. Survival rates in the fjord varied with fork length (L_F), and ranged from 97·0 to 99·5% km^?1. On average, the post‐smolts spent 1·5 days (36 h, range 11–365 h) travelling from the river mouth to the last fjord array, 31 km from the river mouth. The migratory speed was slower (1·8 L_F s^?1) in the first 4 km after sea entry compared with the next 27 km (3·0 L_F s^?1). Post‐smolts entered the fjord more often during the high or ebbing tide (70%). There was no clear diurnal migration pattern within the river and fjord, but most of the post‐smolts entered the fjord at night (66%, 2000–0800 hours), despite the 24 h daylight at this latitude. The tidal cycle, wind‐induced currents and the smolts' own movements seemed to influence migratory speeds and routes in different parts of the fjord. A large variation in migration patterns, both in the river and fjord, might indicate that individuals in stochastic estuarine and marine environments are exposed to highly variable selection regimes, resulting in different responses to environmental factors on both temporal and spatial scales. Post‐smolts in the northern Alta Fjord had similar early marine survival rates to those observed previously in southern fjords; however, fjord residency in the north was shorter.     

Hydropower-related mortality and behaviour of Atlantic salmon smolts in the River Sieg,a German tributary to the Rhine

We studied downstream migration of 256 radio-tagged Atlantic salmon smolts passing a low-head power station where technical facilities have been improved to allow safe migration via several bypass routes. Extra mortality was 7 and 17% (two years) in the power station reservoir, and a minimum of 10 and 13% at the power station compared to in a control stretch. The majority of the smolts followed the main flow at the power station, towards the turbines. Sloped bar racks with 10 mm bar spacing hindered smolts from entering the turbines, hence there was no turbine mortality. Smolts used surface openings in the racks, which directed them to a bypass route outside the turbines. The extra mortality in the reservoir and at the power station was related to physical injuries in bypass routes and to predation. The mortality risk in the reservoir and at the power station decreased with increasing migration speed. Migration speeds increased with water discharge. Migration was slower when the smolts passed the power station than on other stretches. This study shows that hydropower regulation caused elevated mortality and delays for downstream migrating fish, even with improved technical facilities to reduce mortality.     

The North Atlantic subpolar gyre and the marine migration of Atlantic salmon Salmo salar: the ‘Merry‐Go‐Round’ hypothesis

One model for marine migration of Atlantic salmon Salmo salar proposes that North American and southern European stocks (<62° N) move directly to feeding grounds off west Greenland, then overwinter in the Labrador Sea, whereas northern European stocks (>62° N) utilize the Norwegian Sea. An alternate model proposes that both North American and European stocks migrate in the North Atlantic Subpolar Gyre (NASpG) where S. salar enter the NASpG on their respective sides of the Atlantic, and travel counterclockwise within the NASpG until returning to natal rivers. A review of data accumulated during the last 50 years suggests a gyre model is most probable. Freshwater parr metamorphose into smolts which have morphological, physiological and behavioural adaptations of epipelagic, marine fishes. Former high‐seas fisheries were seasonally sequential and moved in the direction of NASpG currents, and catches were highest along the main axis of the NASpG. Marking and discrimination studies indicate mixed continental origin feeding aggregations on both sides of the Atlantic. Marked North American smolts were captured off Norway, the Faroe Islands, east and west Greenland, and adults tagged at the Faroes were recovered in Canadian rivers. Marked European smolts were recovered off Newfoundland and Labrador, west and east Greenland, and adults tagged in the Labrador Sea were captured in European rivers. High Caesium‐137 (¹³⁷Cs) levels in S. salar returning to a Quebec river suggested 62·3% had fed at or east of Iceland, whereas levels in 1 sea‐winter (SW) Atlantic Canada returnees indicated 24·7% had fed east of the Faroes. Lower levels of ¹³⁷ Cs in returning 1SW Irish fish suggest much of their growth occurred in the western Atlantic. These data suggest marine migration of S. salar follows a gyre model and is similar to other open‐ocean migrations of epipelagic fishes.     

The effect of stocking with 0+ year age‐class Atlantic salmon Salmo salar fry: a case study from the River Bush,Northern Ireland

An enhancement programme based on stocking 0+ year age‐class Atlantic salmon Salmo salar, conducted in the River Bush, Northern Ireland, U.K. over the period 1996–2005, was reviewed with reference to the performance and biological characteristics of wild fish. Wild ova to 0+ year fry (summer) survival was c. 8% with subsequent wild 0+ year fry‐to‐smolt survival c. 9%. Stocked unfed 0+ year juveniles gave c. 1% survival to smolt whilst fed 0+ year S. salar stocked in late summer exhibited survival at c. 5%. Stocking with unfed and fed fry contributed to increased smolt production and helped attain local management objectives between 2001 and 2005. Significant differences in biological characteristics were observed between wild and stocked‐origin fish. Wild‐smolt cohorts were dominated by 2+ year age‐class fish on the River Bush whilst smolts originating from fed fry mostly comprised younger 1+ year individuals. The mean mass of 1+ year smolts derived from stocked fed fry was significantly lower than that of wild 1+ year smolts, although these differences were not evident between older age classes. Differences in run timing between wild smolts and smolts derived from stocked fry were also apparent with the stocked‐origin fish tending to run earlier than wild fish. Although the stocking exercise was useful in terms of maximizing freshwater production, concerns over the quality of stocked‐origin recruits and the long term consequences for productivity are highlighted.     

Estuarine survival and migratory behaviour of Atlantic salmon Salmo salar smolts

To estimate mortality rates, assess the spatio‐temporal dynamics of natural mortality and examine migratory behaviour during the fresh to saltwater transition, 185 wild Atlantic salmon Salmo salar smolts were implanted with coded acoustic transmitters. Seaward migration of tagged S. salar from four river systems in an area of Nova Scotia, Canada known as the Southern Upland was monitored using fixed receivers and active telemetry over 3 years. Cumulative survival through the river, inner estuary, outer estuary and bay habitats averaged 59·6% (range = 39·4–73·5%). When standardized to distance travelled, survival rates followed two patterns: (1) constant rates of survival independent of habitat or (2) low survival most frequently associated with inner estuary habitats. In rivers where survival was independent of habitat, residency periods were also independent of habitat, post‐smolts exhibited few upstream movements, took a more direct route to the ocean and reached the ocean rapidly. Alternatively, in rivers where survival was habitat specific, residency was also habitat specific with overall increased residency, more frequent upstream movements and delayed arrival to the open ocean. The sudden disappearance of most (75–100%) smolts and post‐smolts assumed dead during the course of this study warrants further examination into the role of avian predators as a mortality vector.     

Fecundity regulation by atresia in turbot Scophthalmus maximus in the Baltic Sea

Down‐regulation of fecundity through oocyte resorption was assessed in Baltic Sea turbot Scophthalmus maximus at three locations in the period from late vitellogenesis in April to spawning during June to July. The mean ± s.d . total length of the sampled fish was 32·7 ± 3·1 cm and mean ± s.d . age was 6·2 ± 1·5 years. Measurements of atresia were performed using the ‘profile method’ with the intensity of atresia adjusted according to the ‘dissector method’ (10·6% adjustment; coefficient of determination was 0·675 between methods). Both prevalence (portion of fish with atresia) and intensity (calculated as the average proportion of atretic cells in fish displaying atresia) of atresia were low in prespawning fish, but high from onset of spawning throughout the spawning period. Atretic oocytes categorized as in early alpha and in late alpha state occurred irrespective of maturity stage from late prespawning individuals up to late spawning fish, showing that oocytes may become atretic throughout the spawning period. Observed prevalence of atresia throughout the spawning period was almost 40% with an intensity of c. 20%. This indicates extensive down‐regulation, i.e. considerably lower realized (number of eggs spawned) v. potential fecundity (number of developing oocytes), suggesting significant variability in reproductive potential. The extent of fecundity regulation in relation to fish condition (Fulton's condition factor) is discussed, suggesting an association between levels of atresia and fish condition.     

Molecular analysis of the bacterial communities in the live Pacific oyster (Crassostrea gigas) and the influence of postharvest temperature on its structure

Aims: To evaluate the effect of postharvest temperature on bacterial communities in live Pacific oysters (Crassostrea gigas) using nonculture‐based methods. Methods and Results: Live oysters were compared before and after storage at 4, 6, 15, 20 and 30°C using terminal restriction fragment length polymorphism (T‐RFLP). Bacterial communities in freshly harvested (control) vs stored oysters were significantly different. Changes in bacterial communities at 4, 15 and 30°C observed by T‐RFLP were further investigated by clone library analysis. Members of the Proteobacteria predominated (43·0–57·0% of clones) in control oysters, while storage altered the bacterial profile. At 4°C, Psychrilyobacter spp. (phylum Fusobacteria) predominated (43·8% of clones), while at 15 and 30°C, members of the phylum Bacteroidetes represented 63·0 and 60·2% of clones, respectively. High microbial diversity in oysters was observed, with at least 73 different genera‐related clones among all samples. Conclusions: Changes in the overall bacterial community of Pacific oysters were influenced by storage temperature and would likely not be detected by standard culture‐based methods currently used to assess oyster quality. Certain dominant genera, such as Psychrilyobacter, Polynucleobacter and a bacterial group related to Alkaliflexus, should be further studied as possible indicators for postharvest temperature control. Significance and Impact of the Study: This work is the first report describing the effect of different storage temperatures on bacterial diversity in postharvest live Pacific oysters using molecular‐based methods.     

Riverine and near coastal migration performance of hatchery brown trout Salmo trutta

To study migration performance and return rates of hatchery brown trout Salmo trutta smolts the first 5 months after release, 50 fish in each year (fork length, L_F, 158–288 mm) were in two subsequent years tagged with acoustic transmitters and recorded by automatic listening stations in the River Nidelva (central Norway), its estuary and in the marine environment. More than half of the smolts became anadromous migrants (52% in 2011 and 70% in 2012). The fish spent longer time in the estuary than in the marine environment and the results suggest that migratory behaviour of S. trutta smolts is not only restricted to be resident or anadrome–lacustrine, but that there is also an intermediary strategy of estuarine feeding. There were no differences in L_F or mass between groups of smolts with different migration patterns. Return rates from the sea within the first 5 months after release were in both years 16%. Median progression rate in the river was 0·090 L_F s^?1 but decreased significantly as the smolts entered the estuary (0·015 L_F s^?1). The long residential time in the estuary may increase the risk of negative effects of anthropogenic activities in estuaries, such as harbours and industrial development, and special attention should be given to evaluate effects of such activities.     

Extensive feeding on sockeye salmon Oncorhynchus nerka smolts by bull trout Salvelinus confluentus during initial outmigration into a small,unregulated and inland British Columbia river

Stomach contents were collected and analysed from 22 bull trout Salvelinus confluentus at the edge of the Chilko Lake and Chilko River in British Columbia, Canada, during spring outmigration of sockeye salmon Oncorhynchus nerka smolts. Twenty of the 22 (>90%) stomachs contained prey items, virtually all identifiable prey items were outmigrant O. nerka smolts and stomach contents represented a large portion (0·0–12·6%) of estimated S. confluentus mass. The results demonstrate nearly exclusive and intense feeding by S. confluentus on outmigrant smolts, and support recent telemetry observations of high disappearance rates of O. nerka smolts leaving large natural lake systems prior to entering high‐order unregulated river systems.     

Does size matter? A test of size‐specific mortality in Atlantic salmon Salmo salar smolts tagged with acoustic transmitters

Mortality rates of wild Atlantic salmon Salmo salar smolts implanted with acoustic transmitters were assessed to determine if mortality was size dependent. The routinely accepted, but widely debated, ‘2% transmitter mass: body mass’ rule in biotelemetry was tested by extending the transmitter burden up to 12·7% of body mass in small [mean fork length (L_F) 138·3 mm, range 115–168 mm] downstream migrating S. salar smolts. Over the short timescale of emigration (range 11·9–44·5 days) through the lower river and estuary, mortality was not related to S. salar size, nor was a relationship found between mortality probability and transmitter mass: body mass or transmitter length: L_F ratios. This study provides further evidence that smolt migration studies can deviate from the ‘2% rule’ of thumb, to more appropriate study‐specific measures, which enables the use of fishes representative of the body size in natural populations without undue effects.     

Physiological preparedness and performance of Atlantic salmon Salmo salar smolts in relation to behavioural salinity preferences and thresholds

This study investigated the relationships between behavioural responses of Atlantic salmon Salmo salar smolts to saltwater (SW) exposure and physiological characteristics of smolts in laboratory experiments. It concurrently described the behaviour of acoustically tagged smolts with respect to SW and tidal cycles during estuary migration. Salmo salar smolts increased their use of SW relative to fresh water (FW) from April to June in laboratory experiments. Mean preference for SW never exceeded 50% of time in any group. Preference for SW increased throughout the course of smolt development. Maximum continuous time spent in SW was positively related to gill Na⁺, K⁺‐ATPase (NKA) activity and osmoregulatory performance in full‐strength SW (measured as change in gill NKA activity and plasma osmolality). Smolts decreased depth upon reaching areas of the Penobscot Estuary where SW was present, and all fish became more surface oriented during passage from head of tide to the ocean. Acoustically tagged, migrating smolts with low gill NKA activity moved faster in FW reaches of the estuary than those with higher gill NKA activity. There was no difference in movement rate through SW reaches of the estuary based on gill NKA activity. Migrating fish moved with tidal flow during the passage of the lower estuary based on the observed patterns in both vertical and horizontal movements. The results indicate that smolts select low‐salinity water during estuary migration and use tidal currents to minimize energetic investment in seaward migration. Seasonal changes in osmoregulatory ability highlight the importance of the timing of stocking and estuary arrival.     

Norwegian spring‐spawning herring as the test case of piecewise linear regression method for detecting maturation from growth patterns

Data from Norwegian spring‐spawning herring Clupea harengus, were used empirically to assess the performance of a regression method aimed at detecting the time of the onset of maturation from growth trajectories. At the level of the whole dataset, the piecewise linear‐regression method was accurate and showed only a minor bias (?0·17 years) relative to age at maturation visually read from scales. The method, however, was relatively imprecise and provided an estimate of age at maturation equal to the one read from the scale in less than half of the cases (47·6%). Moreover, bias was strongly dependent on age at maturation: the age at maturation of early maturing fish was often overestimated, whereas the opposite was true for late‐maturing fish. Accuracy and precision of the regression method relative to visual readings also depended on the growth type (determined by the nursery area of young C. harengus) and cohort but not on sex. Modifying the original regression method enabled marginally improving the precision of the approach but a strong maturation age‐dependent bias persisted. The results with C. harengus suggested that age‐at‐maturation estimates from the piecewise linear regression method should be treated with caution.